This invention relates to refractor optical systems and, more particularly, to a spherical and a cylinderical lens series for a refractor having regular additive errors to permit non-additive combinations for correction of additive error.
Refractors are well known ophthalmic instruments used for determining the proper lens value necessary to correct the defective vision of a patient. The refractor typically includes a right hand battery and a left hand battery, each for enabling the practitioner to place various corrective lenses in alignment with a respective patient's eye. Each of the batteries is alike and each includes a series of spherical lenses and series of cylindrical lenses. The spherical lens assembly and the cylindrical lens assembly each conventionally include a pair of discs. One of each pair of discs contains lenses having a "weak" power in incremental increases of power and the other disc of each pair contains lenses having "strong" power. The lenses of the "strong" power disc are of sequentially increasing power with the increments of increasing power usually at least being equal to the highest power of the corresponding "weak" lens plus the "weak" lens increment. The practitioner may rotate one of the lenses in each disc into alignment with the patient's eye. In early refractors, like trial lens sets, the practitioner could select the particular combination of lenses which he wished to place before the patient's eyes. Each lens was manufactured to have a specific power whether the lens was used by itself or in combination with one or more other lenses. For example, to provide a cylinder power of plus 6.50 diopters, the practitioner would select the +6.0 diopter sphere lens and the +0.5 diopter cylinder lens. In lens series of such prior art devices, the designer attempted to minimize the error in any particular combination since each lens, for example the +0.25 diopter cylinder lens, was used any time the practitioner needed to add 0.25 diopter of cylinder. In the average trial lens sets there are over three thousand lens combinations in which the 0.25 diopter cylinder lens may be used. Since the only way to correct for additive error was to refer to a reference chart which covered all the permutations of lens combinations, the lens designer attempted to minimize the error resulting from combining lenses in each possible combination of the series.
As used hereinafter, the term "additive error" refers to the difference between the real cylinder power of a lens combination and the power expected from the sum of the powers of the individual cylinder lenses in the combination. The term "significant" additive error is used herein to denote additive error in excess of 0.1 diopter.
As refractors developed and became more refined, the disc containing the "weak" lenses were mechanically coupled to the disc containing the "strong" lenses. This permitted the practitioner to operate a single control to vary the spherical power presented to the eye and similarly a single control to vary the cylinder power presented to the eye. The constrictions placed upon lens design were even more severe in such instruments because the mechanical linkage used to couple the cylinder lens discs prevents the practitioner from personally selecting individual lens combinations.